The present invention pertains to blends of ethylene vinyl acetate (EVA) and ethylene-styrene interpolymers (ESI). In one embodiment, the present invention pertains to foams prepared from such blends. In another embodiment, the present invention pertains to such foams that are cross-linked and to products prepared from such cross-linked foams. In yet another embodiment, the present invention relates to a process for making such foams and products.
One embodiment of cross-linked foam products include footwear applications. Such foams must meet a variety of performance requirements, but usually the enhancement of any given performance attribute detracts from the performance of another attribute. For example, conventional foams prepared from substantially linear ethylene polymers, or ethylene vinyl acetate (EVA) polymers, or a blend of the two, often require a tradeoff between foam density, compression set resistance and rebound resilience. Low foam density is desirable from both cost and performance perspectives, but compression set resistance decreases with decreasing foam density, and foam shrinkage increases as a result of the high foam expansion needed to achieve a low foam density.
Low compression set, on the other hand, is desirable to maintain cushioning over the life of a footwear article (e.g., shoe, sandal, etc.), but low compression set usually requires higher foam density, increased polymer crystallinity, and increased cross-link density. However, increasing polymer cyrstallinity decreases rebound resilience and produces a "harder" foam. In addition, polymer density has a practical upper limit because most footwear foams must remain sufficiently thermoplastic for thermoforming into a finished shape. Moreover, high cross-link levels increase foam shrinkage.
High rebound resilience is desirable in footwear foams to maximize energy return to the wearer. Rebound resilience is maximized by low crystallinity and highly cross-linked polymers, but both of these attributes contribute to high foam shrinkage.